Patent GB-1049093-A (GB)
Title: Television video signal modifying apparatus
IPC: H04N5/18
Filed: 1963-07-11
Abstract: 1,049,093. Television. FERNSEH G.m.b.H. July 11, 1963 [July 11, 1962], No. 27459/63. Heading H4F. A video signal Ia, Fig. 2, comprising picture components during the intervals t and blanking components during the intervals t<SP>1</SP>, is applied from terminal 1, Fig. 1, via transistor stages 7 and 15 to an output terminal 36 and has the peak level in one or the other direction (i.e. the level corresponding to the brightest or darkest picture area) held at a constant potential by a transistor clamp stage 26 which is rendered conductive by the inverted video signal IIa which is obtained from the collector of transistor 15 and applied to the base of transistor 26 via an emitter-follower stage 21. Where transistor 26 is a PNP type, the stage may be rendered conductive by the most negative extremity of signal IIa so that the peak level 2 corresponding to the brightest picture area in the original signal is held at a constant potential. The output signal is then as shown at IIIa with level 2 stabilized at potential p<SP>1</SP>. Where transistor 26 is an NPN type, the circuit functions to stabilize the level 4 corresponding to the darkest area so that the output is as shown at IVa with level 4 held at potential P. Emitterfollower 21 may be dispensed with, the signal which is applied to the base of transistor 26 being obtained directly from the collector of transistor 15, from transistor 7 or from a stage (not illustrated) which precedes stage 7. In a modified arrangement, Fig. 3 (not shown), for use where the signal level during blanking exceeds the peak level due to the darkest picture area, a gate stage is included in the signal path between transistor 15 and the clamp stage and is controlled by pulses coincident with the blanking intervals so as to substitute peak levels in the " white " direction. The clamp stage is an NPN type and thus responds to stabilize the peaks corresponding to the darkest picture area without interference by the blanking signals. The stabilized video signal may be applied to a limiter circuit, Fig. 5 (not shown), which may be operated to separate either the blanking (and any associated synchronizing) signals or picture signals with a lift or pedestal by which the darkest point exceeds the blanking level set by the clipping level. The stabilized signal may also be applied to a circuit Fig. 6 (not shown), which incorporates a gate stage which is opened only during the blanking interval and is connected to a capacitor. Depending upon the peak level which is stabilized the circuit functions to produce a control potential varying in accordance with the difference R<SP>1</SP> (curve XI, Fig. 4) between the darkest picture point and the blanking level, or the difference R<SP>11</SP> (curve XII, Fig. 4) between the brightest picture point and blanking level. In the first case the control potential may be employed to control the limiting potential of a clipper stage; in the second case the control potential may be employed for gain control or control of the characteristic of a non-linear amplifier. An application of the invention is illustrated in Fig. 7 where an original signal, curve V, Fig. 4, has its darkest picture point 46 stabilized at a reference level 60 by a circuit 51, which functions as described above in connection with Fig. 3 (not shown), and is then applied as the reference potential for a clamp circuit 92 which is controlled by pulses, curve XV, falling within the blanking interval and functions to stabilize the level of the original signal in a blanking amplifier 91. The resulting signal which is reblanked throughout the whole of the blanking interval, appears as shown in curve XIV with the darkest point 46 held a constant level P and is applied to limiter stage 95 to yield a final signal, curve X, in which the blanking level always differs from the darkest picture point by a fixed potential 55. A detailed transistor circuit for carrying out this application of the invention is described with reference to Fig. 8 (not shown). In another system, Fig. 9, the original signal, curve V, Fig. 4, is applied to a mixer stage where impulses, curve XVI, from a generator 121 are added to the signal to produce a composite signal as shown in curve XVII. This signal is then applied via non-linear amplifiers 122, 123 to a clamping stage 124 which stabilizes the signal level at a time during the injected impulses. The signal from amplifier 123 is also applied to a level stabilizing circuit as described above with reference to Fig. 3 (not shown) and is then applied to a circuit 126 which develops a control potential dependent on the difference R<SP>1</SP>, curve XVIII, between the stabilized darkest picture points 46 and the base of the blanking signals. The control potential controls the amplitude of the impulses from generator 121 in such manner that the tips of these have the same potential as the darkest picture points. Clamping stage 124 thus effectively stabilizes the level of the darkest picture points. Fig. 10 illustrates a further system in which a video signal, curve V, is blanked in a circuit 132 to yield a signal, curve XX, which is limited in a stage 135 to produce a final output signal as shown in curve X. The final signal is also applied to a stabilizing circuit as described above with reference to Fig. 3 (not shown) which feeds a circuit 135 which functions to produce a control signal proportional to the difference R<SP>1</SP>, curve XXI, between the stabilized darkest picture points and the blanking level. The control voltage is applied to limiter 133 to vary the limiting level so as to maintain constant the difference 55, curve X, between the darkest picture points and the blanking impulses. The original signal, curve V, may alternatively be applied directly to circuit 51 as indicated by the broken line connection.